Embodiments of the invention relate to an array substrate and a driving method thereof.
Currently, thin film transistor liquid crystal displays (TFT-LCDs) are widely used flat panel displaying devices. A TFT-LCD comprises an array substrate and a color filter substrate, which are bonded together with liquid crystal interposed therebetween. Liquid crystal is driven to be rotated by an electrical field formed between the array substrate and the color filter substrate, thus the light transmissivity is changed and different grey levels or different images are presented.
A top view illustrating a part of a pixel unit in a conventional TFT-LCD array substrate is shown in FIG. 1. Gate lines 3 and data lines 2 are crossed with each other on a base substrate 1 so as to define a plurality of pixel units in a matrix. A pixel electrode 9 and a TFT drive switch are arranged in each pixel unit. Generally, common electrode lines 10 are further disposed on the array substrate. For clarity, insulating layers (such as a gate insulating layer and a passivation layer) formed between the conductive structures are not shown in the drawing. The TFT drive switch may comprise a first gate electrode 4, a first active layer 6, a first source electrode 5, and a first drain electrode 7. The first drain electrode 7 is connected with the pixel electrode 9 via a first via hole 8. When a turning-on (“ON”) voltage of a higher potential is applied on the first gate electrode 4, the first source electrode 5 is electrically communicated with the first drain electrode 7; when a turning-off (“OFF”) voltage of a lower potential is applied on the first gate electrode 4, the source electrode 5 is disconnected with the first drain electrode 7. During operation, the “ON” voltage is applied to the gate line 3 so as to turn on the TFT drive switch in each pixel unit, and then the data line 2 applies an image voltage signal to the pixel electrode 9 in each pixel unit through the TFT drive switch.
The color filter substrate also comprises a base substrate on which a black matrix is formed for blocking the light-proof regions such as the regions where the data lines, the gate lines and the TFT drive switches are located. A common electrode is formed on the black matrix, and a common electrode line supplies a common voltage to the common electrode. The common voltage applied on the common electrode and the image signal voltage applied to the pixel electrode in a pixel unit form an electrical field, which controls the deflection of the liquid crystal molecular and further controls the intensity of the transmitted light, i.e., the light transmissivity of the pixel unit.
A field sequential color (FSC) type TFT-LCD is one kind of TFT-LCDs. Typically, for the FSC type TFT-LCD, no color filter resins are provided on the color filter substrate; however, the color of the image is presented by illuminating of back lights of different colors in a time-sharing manner, that is to say, an image frame is divided into three image sub-frames (or three image fields). Each image sub-frame starts when the data line begins to input the data voltage signal and ends when the data line begins to input the data voltage signal for the next image sub-frame. In the three image sub-frames, three homochromous back lights of red, green and blue colors are illuminated sequentially. Since the alternation frequency of the image sub-frames is high, the human beings will not feel the time-sharing display of the three colors but see the colorful image frame after color mixing.
FIG. 2 is a schematic view of a drive waveform over time of a conventional FSC type TFT-LCD. The axis of abscissa is a time axis. According to the up-to-down sequence, the three rows of waveforms in the first group are waveforms of the drive voltages in the gate lines, which are at an “ON” voltage in the case of a higher potential. FIG. 2 schematically shows the drive voltage waveforms in the first row, the (N/2)th row and the Nth row, where N is a natural number and the row number of the total pixel units in the array substrate. The drive voltage variation pattern of other unspecified rows is similar to those mentioned above. In the example, the description is made in the case “N” is an even number; however, the number of the gate lines may be an odd number. The row of waveform in the second group is the image voltage signals in one data line. FIG. 2 only shows the image voltage signals in the data line of a certain column when an “ON” voltage is inputted into the gate lines in the first row, the (N/2)th row and the Nth row. The image voltage signals for the pixel units in each row may be different. Three rows of waveforms in a third group are the light transmissivity variation waveforms for the pixel units in the first row, the (N/2)th row and the Nth row under the driving of the above drive voltages and image voltage signals. The row of waveform in the fourth group is a waveform for driving to light the back lights. Of two rows in the fifth group, the first row represents the cycle time of each image sub-frame, and the second row represents the cycle times for three image sub-frames of red, blue and green for each image frame. As shown in FIG. 2, each image sub-frame comprises three periods of time. Taking the red image sub-frame as an example, it comprises a row scanning period Ta, a response period Tb and a back light lighting (“ON”) period Ton. The row scanning period Ta is the time period in which a line-by-line scanning is performed for all the gate lines on the array substrate so as to realize a pixel electrode display refresh, which starts when the gate line in the first row inputs an “ON” voltage and ends when the input of an “ON” voltage from the last row of gate lines is completed. One line scanning refers to that an “ON” voltage is inputted over one of gate lines so that the image voltage signals in the data lines can be inputted. The display refresh refers to that the pixel electrode voltage in one pixel unit is changed, which renders the display of the pixel unit changed. When the display across the whole array substrate is changed, an image refresh occurs. The response period Tb refers to the time period in which the liquid crystal completes its rotation under the influence of the electrical field and which starts when the row scanning period Ta ends and ends when the liquid crystal corresponding to the last row has responded (i.e., has completed the rotation). The back light “ON” period Ton is the time period in which the back light of the color corresponding to the image sub-frame is in a lighting state, which starts when the response period Tb ends and ends when the next image sub-frame begins.
The back lights should be lighted intermittently for the following reasons. The colors to be presented in two adjacent image sub-frames are different, and the line-by-line scanning on the gate lines and line-by-line refresh on the content of the pixel units need a certain period of time, i.e., the sum of the row scanning period Ta and the response period Tb. If the back light keeps in a lighting state in the row scanning period Ta and the response period Tb, then a mix of the image colors may occur because the display refresh has not been performed for a part of pixel units on the whole array substrate. For example, when the liquid crystal in the last row of pixel units has not been refreshed or rotated to a predetermined position, the color of the former image sub-frame is still presented. If the back light for the current image sub-frame is lighted at this time, then the display corresponding to the rotation angle of the liquid crystal does not match the back light, and improper image displaying will occur.
It can be known from the above that the conventional FSC type TFT-LCD uses a row scanning display refresh driving method, that is to say, at the same time, the image signal voltage in only one row of pixel units and the corresponding display are refreshed. In order to avoid the color mixing, the back light must be turned off temporarily. After the image refresh of the current image sub-frame is completed and the liquid crystal has responded, the corresponding homochromous back light is lighted. Therefore, there is a disadvantage of low utilization efficiency of the back light, and the back light only can be illuminated in a part of time period for each image sub-frame. In the conventional FSC type TFT-LCD, since the row scanning period Ta and the liquid crystal response period Tb for the display refresh of each image sub-frame occupy a large part of the cycle time of an image sub-frame, and the back light “ON” period Ton is reduced dramatically, which renders the reduced utilization efficiency of the back light accordingly. Thus, the image brightness of the LCD is decreased, or the power consumption and the cost are increased in order to enhance the brightness. The shortcoming become more prominent in the case of a high frame frequency and a high resolution display, i.e., in the case where the image sub-frame cycle time is short and the row scanning period Ta is long.
FIG. 3 shows an equivalent circuit diagram of a pixel unit in a conventional array substrate.